The present invention relates to enhancing the operation of a catalytic converter and, in particular, to a process and apparatus for accelerating the rate of heating of a fixed bed catalyst to the light-off temperature for a catalytic reaction by supplying supplementary energy from an external energy source for a limited time period. A fixed bed catalyst within the converter may be present in a solid body or in a bulk form. The catalytic carrier within the converter has an exhaust gas inlet area defined by a cross-section of the catalyst carrier bed flow channels.
A typical field of use for such a process is catalysis of car exhaust gases. Modern three-way catalytic car exhaust gas converters usually consist of catalytically active coatings on a honeycomb-structured carrier made of ceramic or metal. Such converters can convert up to 90% of harmful substances contained in car exhaust gases, such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO.sub.x), into the non-toxic substances water, carbon dioxide and nitrogen. Such conversion, however, only takes place after a catalyst within the converter has reached its particular operating temperature (typically between 400.degree. and 1000.degree. C.) and that the exhaust gas has a correct chemical composition as a result of proper regulation of the fuel/air ratio.
During the initial stages of a cold start, however, the harmful substances may pass through the still inactive catalyst without being converted. When a so-called "light-off" temperature is reached, conversion of approximately 50% of the harmful substances takes place. The light-off temperature is dependent on the catalyst formulation and on the type of harmful substance(s) being converted. In the case of car exhaust catalysis, the light-off temperature is typically in the range of 250.degree. to 400.degree. C. The duration of the cold start phase, between the initial start-up and achieving the light-off temperature, usually takes about 200 to 300 seconds.
Depending on the nature of a particular use (i.e., long or short-distance driving) harmful substance emissions during the cold start phase may constitute the majority of the total harmful vehicle emissions for a particular use. Hence, it is advantageous to heat the catalyst as quickly as possible. Passive and active techniques have previously been proposed for accelerating the heating of a catalyst in order to reduce emissions. Passive measures include locating the catalyst as close as possible to the internal combustion engine, insulating the exhaust gas system against heat losses, and reducing the heat capacity of the exhaust gas system. Proposed active measures have included supplying supplemental energy to the catalytic system during the cold start phase in the form of microwave energy (PCT WO 90/14507) or in the form of electric heat.
Typical systems related to electrically heated catalytic converters are described in SAE publications SAE 930 383 "Reduced Energy and Power Consumption for Electrically Heated Extruded Metal Converters", and SAE 930 384 "Optimisation of an Electrically Heated Catalytic Converter System: Calculations and Application".
In prior electrically heated systems, catalytic elements consisting of a metal honeycomb structure acts as a carrier for a catalytically active coating. An electric current is passed through the metal structure to produce a heating effect. Electric power of 1500 W per 50 g weight of metal honeycomb structure is typically required to heat a catalyst in such a system to a light-off temperature of 400.degree. C. within 10 seconds. In an attempt to reduce the amount of electrical power required, it has been proposed that a catalyst be provided which is designed to be effective only as a start-up catalyst. Such a start-up catalyst heating arrangement is located upstream of a main catalyst portion which performs conversion following start-up.
German Patent Publication DE-OS 40 35 971 describes a heatable catalyst arrangement for the purification of exhaust gas from an internal combustion engine which arrangement consists of a first portion having a part-catalyst, a subsequent electrically heated catalyst portion which is further connected in series to another part-catalyst portion. However, the heated catalytic system according to DE-OS 40 35 971, like all catalytic arrangements which heat the catalyst by passing current through a conductive carrier, is susceptible to deterioration of electrical contacts and conductive paths which occurs during normal operation. Furthermore, depositions on the metallic carrier can lead to corrosion, faulty contacts and short-circuits. Another important problem is that electrically heated catalyst systems draw significant amounts of current, typically between 100 to 500 A, from the vehicle battery. This current drain reduces battery life and requires additional wiring and the corresponding additional complexities and resistance losses relating thereto. In many cases, the high energy requirements of electrically heated catalyst systems necessitates the installation of a second vehicle battery dedicated strictly to the catalyst heating system.
The object of the present invention is to provide a system for accelerating the rate of heating of a fixed bed catalyst by supplying supplemental energy to heat a predetermined portion of a catalyst bed in such a manner that the total energy required to achieve the desired affect is substantially reduced when compared to that required in prior art systems. The present invention also is suitable for use with catalyst carriers comprised of ceramics, or other non-conducting materials.